An image grab taken from a video uploaded on social networks, shows young men in underwear being marched barefoot along a desert road before being allegedly executed on Aug. 27, 2014 by Islamic State (IS) militants at an undisclosed location in Syria's Raqa Province.

Islamic State fighters executed scores of Syrian soldiers captured when the militants seized an airbase in the province of Raqqa at the weekend, according to a video posted on YouTube on Thursday.

The video, confirmed as genuine by an Islamic State fighter, showed the bodies of dozens of men lying face down wearing nothing but their underwear. They were stretched out in a line that appeared to be dozens of meters long.

A separate pile of bodies was shown nearby. Reuters could not independently verify the authenticity of the video.

The caption beneath it said the dead numbered 250. An Islamic State fighter in Raqqa told Reuters via the Internet: "Yes, we have executed them all."

An Islamic State militant uses a bull horn to announce to residents of Tabqa city that Tabqa air base has fallen to Islamic State militants, in nearby Raqqa city, Aug. 24, 2014.

Islamic State, a radical offshoot of al-Qaida, stormed Tabqa airbase on Sunday after days of clashes with the army and said it had captured and killed soldiers and officers in one of the bloodiest confrontations yet between the two sides.

The capture of Tabqa, the Syrian army's last foothold in that area, and apparent parading and killing of large numbers of its soldiers shows how Islamic State has cemented its grip on the north of the country.

The video begins by showing the captives apparently being marched in the desert with their hands behind their heads and watched by armed men. An Islamic State fighter repeatedly shouts out "Islamic State", to which the men reply "It shall remain".

Not an ally

Islamic State controls roughly a third of Syria, mostly areas in the north and east of the country. The United States has launched airstrikes on the same group over the border in Iraq and is considering doing the same in Syria.

The Syrian government, which is shunned by the West, has presented itself as a partner in a war on Islamist extremists.

But Washington, which has built its Syria policy on Assad leaving power, says he is part of the problem. French President Francois Hollande said on Thursday Assad was no ally in the fight against Islamic State.

Syrian warplanes on Thursday hit Islamic State targets in the eastern province of Deir al-Zor, another of its strongholds, in an airtrike that killed some of the group's commanders, the Syrian Observatory for Human Rights said.

The Observatory said the planes struck a building used as an Islamic State headquarters during a meeting of its commanders.

Syrian state TV reported that the army "eliminated more than 10 terrorists" in an attack east of Deir al-Zor military airport, including two men it named as Islamic State leaders in the province, and destroyed 14 armored vehicles.

Syrian state television reported on Sunday that its troops had withdrawn from the base and regrouped but it has not reported any army deaths or captures. It has said Islamic State suffered heavy losses in the battle over the base.

Sectarian insults

Another video posted online appeared to show at least one Syrian soldier being interrogated before a group of other captured men in their underwear, as voices off camera shout sectarian insults.

The soldier identifies himself as an officer and says he is from the Alawite sect, an offshoot of Shi'ite Islam, like Assad and the majority of high-ranking military officers. Islamic State members are Sunni Muslims.

The interrogator shouts insults at the soldier, suggesting Alawites are born out of wedlock. When at one point the soldier briefly looks down at the floor and rubs his eyes, another interrogator throws a metal rod at him, making him flinch.

"How many have you killed? How many have you raped?" the interrogator shouts. The soldier replies: "None. I've been stationed here in the airport."

The interrogator asks why the soldier had been fighting on behalf of Assad and did not defect and he replies that he would have just been sent back to the army.

"They would have sent you right back to the army? And we're going to send you right back to hell: by slaughter," the interrogator says, making him chant Islamic State slogans.

Although it is not clear how widespread public anger in Syria might be about the fall of the air base, some people supportive of the army have expressed anger on social media.

The Islamic State militants aim to set up a trans-border caliphate in the Iraqi and Syrian territory they have captured.

The United States has carried out airstrikes on Islamic State in Iraq and left open the option for similar action in Syria.